As illustrated in FIG. 12, a known piezoelectric diaphragm pump assembly comprises an inlet port 39 with a ball valve 38, an outlet port 34 with a ball valve 38, and bimorphic type acutuators 36 and 37 at both sides of a pump room. When the actuators 36 and 37 are oscillated by applying an electric voltage to the actuator, then volume change of the pump room repeatedly occurs, so that the assembly acts as a pump means. In the bimorphic actuators 36 and 37, a large hysteresis is observed between the displacement and the electric voltage applied, and therefore there is no linear relationship between the displacement and the voltage, as shown in FIG. 13. In the experiment shown in FIG. 14, use was made of an actuator coated with a silicone rubber film.
Generally, a bimorphic actuator may oscillate at a frequency range near the resonance frequncy so that the flow rate is not proportional to the frequency employed. Furthermore, the bimorphic actuator shows a spherical displacement(a) as shown in FIG. 14, so that said acutuator is very poor with respect to the displacement and volume change.
Japanese Patent Application No. 159,778-1987, which was laid open on July 15, 1987, discloses a longitudinally effective type piezoelectric diaphragm pump provided with a piezoelecteic element with a small displacement. So, the flow rate of this pump is rather low.
Japanese Utility Model Application No. 103,580-1986, which was laid open on July 1, 1986, discloses a liquid pump with a plunger and with a lever for increasing the displacement. However, in the case of this pump, it is impossible to attain a high flow rate unless use is made of a diaphragm having a large diameter.